Three lines of work are proposed: 1. Studies of mammalian beta-mercaptopyruvate sulfurtransferase to define its formal mechanism of action and the basis for its regulatory behavior. 2. Isolation and similar studies of mammalian thiosulfate reductase. 3. Utilization of the information obtained in this project concerning the mechanisms and regulatory behavior of rhodanese, beta-mercaptopyruvate sulfurtransferase and thiosulfate reductase to formulate a metabolic rationale for the physiological occurrence of sulfane compounds.